Endothelial cell injury is considered an important initiating event during atherogenesis. Damage to the endothelial lining of blood vessels allows passage of lipoproteins into the vessel wall. In addition, endothelial injury promotes proliferation of smooth muscle cells resulting in vessel thickening. Other investigators have described desquamated endothelial cells in blood after physical or biochemical injury to the endothelium. This project will assess the potential of using flow cytometry to quantify circulating endothelial cells in peripheral blood. Such an assay may serve as an index of vascular injury.